1. Field of the Invention
The present invention relates to a ceramic jointed body consisting of different ceramic molded parts and having a large size and complicated configuration such as a turbine rotor.
2. Related Art Statement
Recently, research has been conducted for utilizing silicon ceramics in engine parts or the like, since silicon ceramics, such as silicon nitride, silicon carbide, sialon or the like, are more stable and less susceptible than metals to oxidation corrosion or creep deformation at high temperatures. For example, radial turbine rotors made of these ceramic materials are lighter and better in thermal efficiency, allowing the operating temperature of engines to be raised higher, as compared with metallic rotors, so that these are drawing attention as a turbocharger rotor, gas turbine rotor, etc. for automobiles.
However, since such a turbine rotor has complicated three-dimensional shaped blades, naturally it is almost impossible to finish by grinding a sintered body of simple shape such as a round rod, square rod or the like into a desired shape and also it is difficult to obtain such a ceramic body having a complicated configuration by only one molding operation.
Other than the above, injection molding methods have been extensively employed for molding the complicated ceramic bodies.
The method of injection molding ceramics is an application of the conventional injection molding method which has been used for molding plastics. Generally, in injection molding a ceramic body ceramic powder is mixed with an organic binder consisting of a binding agent such as polyethylene, polystyrene or the like, wax, and lubricant. The mixture is subsequently heated to plasticize it and then injection molded in a mold. The thus molded body is burned to remove the organic binder and is then isostatically pressed under hydrostatic pressure and sintered to obtain a ceramic sintered body.
It is very difficult to get a thick body by means of injection molding since in the case of injection molding thick bodies, cracks frequently occur in the bodies after removing the organic binder.
Therefore, there have been studies on the method of producing a ceramic turbine rotor in such a manner that a blade part having a complicated configuration and a rod shaped shaft part are individually molded, and subsequently the molded blade part and the molded shaft part are jointed together to obtain the ceramic molded turbine rotor. For example, Japanese Patent Application Laid-open Publication No. 57-88201 of the assignee of the present application discloses a method of producing a ceramic turbine rotor by individually molding the blade part and the shaft part and fitting these molded parts to each other with a ceramic paste interposed therebetween, and sintering the fitted parts to thereby integrally join them. Japanese Patent Application Laid-open Publication No. 61-111976 also discloses a jointed ceramic turbine rotor composed of ceramic molded blade and shaft parts, each of which has a joining portion of dimensions determined in a given relationship relative to each other so that the molded blade and shaft parts are fitted to each other and integrally jointed by sintering without applying a ceramic paste on the jointing surfaces thereof.
Such a ceramic molded blade part and a ceramic molded shaft part can be individually designed corresponding to the properties required for the blade part and the shaft part, respectively. For example, the shaft part can be designed so as to have a larger diameter at its jointing portion, thereby obtaining a jointed turbine rotor having a high mechanical strength sufficient to withstand a load at high rotating speeds of the rotor. Also, the shaft and blade parts can be designed such that the front end of the shaft part is not exposed on the front surface of the blade part. In this case, when the blade part and the shaft part are jointed, these parts can be urged toward each other by a high pressing force, thereby obtaining a good joint strength at the jointed portion. Moreover, the jointed portion between the blade part and the shaft part is not exposed to the atmosphere, so that when the rotor is subjected to an abrupt heat shock, an influence of the heat shock to the jointed portion can be released, and the jointed portion can be prevented from corrosion caused by hot gas.
However, the methods described in Japanese Patent Application Laid-open Publications Nos. 57-88201 and 61-111976 have the disadvantages that when the dimensions of the jointed body vary, a crack occurs sometimes near the jointing area of the jointed body before sintering. Such cracks are hereinafter called jointing cracks.